C64 With Black Screen

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rmzalbar
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Re: C64 With Black Screen

Post by rmzalbar »

Boy, it sure seems unlikely you'd have the same fault on both.. and it happens with both C64s too? Are you sure you have a good cable? You could also use the o-scope to detect the signal on each of the serial lines when you hit RETURN on giving a command, and make sure the same signal you see on the C64 7406 pins is also seen on the 1541 side at the corresponding pin on the VIA.


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Re: C64 With Black Screen

Post by C64Person »

Yes, that’s my thinking too. Both C64’s are doing it, though I did notice that it just says SEARCHING FOR * indefinitely, but with FastLoad it says DEVICE NOT PRESENT ERROR.

The cable is definitely okay, but I’m not sure how to trace signals all the way to the VIA’s. I was able to trace the signal from the 7406 on the C64 to the 7406 on the 1541 (which seemed to be the same), but that’s as far as I got.
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Re: C64 With Black Screen

Post by C64Person »

Okay, I’ve continued trying to troubleshoot this drive, but had no success: it still doesn’t respond to the computer. At this point, I’m not totally sure what else to check, so I’d certainly appreciate some more advice!
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Re: C64 With Black Screen

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I'll open up one of my systems and take a look around at what the communication to the VIA should look like when sending a command. I hope to be able to tell you where to probe and what you should see.
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Re: C64 With Black Screen

Post by C64Person »

That would be really great! Let me know what you find.
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Re: C64 With Black Screen

Post by C64Person »

Any luck checking those signals? No rush of course, but I'm eager to hear what to look for on this board!

Thanks!
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Re: C64 With Black Screen

Post by rmzalbar »

I am sorry I did not get to it yet. I will do this tonight however, because it will be my last chance for at least a few days as I need to pull everything away from the wall for my landlord to start wall and window renovation tomorrow.
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Re: C64 With Black Screen

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OK, let's begin like this:

What we want to do is simply to open a command channel to the drive, close it, and then report whether a successful handshake occurred. This is ideal for a quick test because it involves a very short burst of data in both directions, and doesn't rely on any physical disk operations which would create a long string of data over a significant period of time.

Test without the Fastload cart because that loads custom kernal into the 1541 and C64 which alters the communication protocol. Switch on both units and type in this two-line program:

10 OPEN15,8,15:CLOSE15
20 IF ST AND 128 THEN PRINT "NOT PRESENT"

This tries to open, and then close a command channel to the drive. Then it checks the system status variable to see if it holds 128 for "device not present.", and reports this condition if found. Now each time you RUN this, you will either get just a READY prompt with no output if successful, or NOT PRESENT if unsuccessful.

Here are scope captures of what happens at the pins on the CIA and VIA that communicate with the serial cable on both the C64 and the 1541 end right when you hit ENTER after typing RUN. Set your scope to freeze the display when it triggers, and set it for rising or falling edge depending on whether the signal is active high or active low, which you can see in my screenshots (active low means that it sits above 2 volts normally, and a when there is a signal it gets pulled down to 0 volts. Basically the inverse of a normal signal. The CIA U2 pins 8 and 9 are active low. Set trigger for falling edge.)

C64:
6526 CIA U2
5 ATN OUT (anything "out" is outbound from the CIA)
6 CLK OUT
7 DATA OUT
8 CLK (anything not marked "out" will actually show both the in and outbound signals on the scope)
9 DATA

1541:
6522 VIA UC3
10 DATA IN
11 DATA OUT
12 CLK IN
13 CLK OUT
14 ATNA
17 ATN IN

I'm attaching a series of scope shots in the following posts (can only attach 3 per post.) First is the C64 and the 1541 with both of them on, normal result. Second, the C64 alone with the 1541 turned off, result of running the program is NOT PRESENT. You should see if your results look like mine.
Last edited by rmzalbar on Thu Jun 25, 2020 6:58 am, edited 4 times in total.
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Re: C64 With Black Screen

Post by rmzalbar »

C64 CIA U2 with both the C64 and 1541 turned on
U2_5.jpg
U2_6.jpg
U2_7.jpg
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Re: C64 With Black Screen

Post by rmzalbar »

U2_8.jpg
U2_9.jpg
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